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Traditional strategies for the control of plant pests and diseases include techniques such as crop rotation, complementary planting, cross protection, early detection of disease and eradication, breeding for disease resistance and chemical control. Breeding for disease resistance and chemical control are two of the most commonly utilised strategies for the control of plant pests and diseases such as insects, fungi, nematodes and viruses.
Chemical control methods in particular are frequently used to control insects and fungi that affect plants, and also nematodes. However, chemical agents are often expensive, and furthermore there are safety concerns relating to the potential impact of the chemical agent on the environment and the use of chemical agents on, for example, fruits and vegetables to be consumed by people. These last issues are factors that have contributed to the growth of the market for organic fruit and vegetables.
Viruses are more difficult to control, and there are very few, if any, commercially available agents for the treatment of plant viruses. Nevertheless, the impact of plant viruses on crops can be significant. For example, in southeast Asia, infection of rice with Rice tungro virus leads to an estimated annual economic loss of $1.5 billion annually. Furthermore. Tomato spotted wilt virus infects a wide variety of plants including tomato, peanuts and tobacco, leading to estimated annual worldwide losses of about $1 billion.
A natural plant defence mechanism against viruses in plants is RNA interference (RNAi), also known as RNA silencing. There is growing evidence that this mechanism plays an important role in natural plant defences against parasites, viruses, insects, nematodes and fungal infections, as well as transposon activity. Through this mechanism exogenous or endogenous double-stranded RNA (dsRNA) is diced into small interfering RNA (siRNA), which is then incorporated into an RNA-induced silencing complex (RISC). The active RISC then uses the siRNA to detect and degrade targeted viral messenger RNA (mRNA), thereby giving rise to antiviral defence.
Plants may be bred to take advantage of naturally occurring disease resistance to various viruses and other organisms. Alternatively, transgenic plants have been produced which employ RNAi to provide resistance to various viruses. Examples of transgenic plants include the Rainbow™ and Sun Up™ papaya (Carica papaya) cultivars which are resistant to the Papaya Ringspot Virus type W. However, there are only a limited number of transgenic RNAi plants commercially available, and large-scale application of transgenic plants has encountered resistance from the public and from regulatory agencies. Furthermore, the cost of developing transgenic plants makes this a laborious and unattractive option.
The RNAi mechanism also may be induced through the exogenous application of dsRNA. However, there has been little research on non-transgenic RNAi approaches to protection of plants. Research that has been performed has illustrated limitations in this approach. For example, in one study topically applied dsRNA could not be detected 7 days post application. Furthermore, when the dsRNA was applied 24 hours after viral infection, the dsRNA was not able to protect the plants (Tenllada and Diaz-Ruiz (2001)). Further studies have illustrated that dsRNA was able to protect N. benthamiana when challenged 5 days after spraying, but a delay of 7 days between spray and virus inoculation could not protect the plant from becoming systemically infected (Tenllado et al, (2003) and Gan et al, (2010)). One factor that impacts on the instability of dsRNA in the environment is ultraviolet light which catalyses the breakdown of dsRNA.
Consequently, there is a need to provide an effective alternative approach for the agricultural control of plant viruses, parasites, insects, nematodes or fungal infections, and especially an approach which at least partially overcomes at least one of the abovementioned disadvantages or which provides the consumer with a useful or commercial choice.